A microwave sensor that detects a human body (intruder) by transmitting microwaves toward a detection area and receiving waves reflected off of the human body (microwaves modulated by the Doppler Effect) is known as an example of a conventional crime prevention device (for example, see Patent Document 1).
Furthermore, one type of microwave sensor that has been proposed involves measuring the distance to the detection target object, such as a human body, present in the detection area by using a plurality of types of microwaves of different frequencies. This type of sensor is configured such that, for example, two types of microwaves of different frequencies are transmitted toward the detection area and the phase difference between two IF signals is detected based on the respective reflected waves. This phase difference correlates to the distance to the detection target object, so that there is a tendency for greater phase differences to occur as distance to the detection target object increases. That is, it is possible to measure the distance to the detection target object by obtaining this phase difference. Furthermore, it is also possible to determine whether or not the detection target object in the detection area is moving by recognizing a change in the phase difference over time. This allows the detection of, for example, only detection target objects that are moving in the detection area.
However, problems such as the following arise when using this type of sensor as a crime prevention sensor and recognizing a change in the phase difference over time in order to recognize only the detection target object moving in the detection area. For example, when this type of sensor is installed outdoors, the sensor may generate a false alarm by erroneously detecting a tree, a bush, or the like swaying in the wind as a detection target object. Similarly, when this type of sensor is installed indoors, the sensor may generate a false alarm by erroneously detecting, as a detection target object, an object other than a human body (a non-detection target object), such as rotational movement of a ventilation fan, blinds or curtains swaying in the wind, or even vibration of the microwave sensor itself.
In view of the above, the present inventors have already proposed techniques for avoiding false alarms by accurately distinguishing between detection target objects such as human bodies and non-detection target objects such as objects other than human bodies (see Patent Document 2).
These proposals involve measuring an amount of change per unit of time in the relative distance to an object that is present in a detection area based on the respective reflected waves, and determining that the object is a detection target object only if the amount of change is not less than a predetermined threshold. More specifically, in contrast to the slight movement distance of a bush or tree swaying in the wind or a rotating fan, the movement distance is large for a detection target object such as a human body, and it is therefore possible to precisely determine whether or not the object is a detection target by recognizing this difference. It should be noted that in the following description, these false alarm countermeasures are referred to as “bush/tree countermeasures” and the aforementioned threshold is referred to as “bush/tree countermeasure level”.
Patent Document 1: JP H07-37176A
Patent Document 2: JP 2003-207462A